Open-loop front-wheel steering provides a certain amount of front-wheel steering assist depending on the amount of front-wheel steering provided by the vehicle operator. Closed-loop front-wheel steering provides feedback in the event that the vehicle is not following the steering path commanded by the vehicle operator. For example, slippery road conditions may prevent the vehicle from turning in the desired direction because the wheels may slip along the road surface. Further, the vehicle's rear quarter may “fish-tail,” also providing a different turn angle than was intended. Closed-loop front-wheel steering assist systems sense the actual vehicle yaw rate and the commanded yaw rate, and generate a gain signal that provides the steering assist by the front wheels if the vehicle yaw rate and the commanded yaw rate are not the same. In these types of active front-wheel steering control systems, the open-loop control is always active and the closed-loop control is only activated if enhanced handling and stability are desired.
Vehicles are designed so that the vehicle handling response complies with certain design specifications. Vehicle dynamic parameters define the vehicle handling response, where nominal parameters define a nominal vehicle handling response. The vehicle dynamic parameter of understeer coefficient is the most dominant vehicle dynamic parameter used in an open-loop control system for determining the stability and dynamic handling behavior of a vehicle. The understeer coefficient defines the vehicle yaw rate or turning radius for a particular steering angle. This parameter varies according to different vehicle loading, tire pressure, tire wear, and vehicle-to-vehicle variations of suspension characteristics, etc.
The control gains for front-wheel steering control systems, including both open-loop and closed-loop control, are optimally determined based on “out-of-factory” vehicle dynamic parameters. During operation of the vehicle over its lifetime, however, the factory-tuned vehicle parameters change as a result of short-term vehicle variations, such as vehicle loading and tire pressure changes, and long-term variations, such as tire wear and vehicle suspension age. When the vehicle dynamic parameters change from their original settings, the original steering control gains are no longer optimal, resulting in a different vehicle handling feel and vehicle stability degradation.
U.S. patent application Ser. No. 10/863,956, titled Real-Time Vehicle Dynamics Estimation System, filed Jun. 9, 2004, assigned to the assignee of this application and herein incorporated by reference, discloses a system that determines vehicle parameter estimates as a vehicle ages. It would be desirable to use such estimated dynamic parameters in an open-loop front-wheel active steering system to improve the open-loop steering gain as the vehicle ages and is subjected to different conditions. By modifying the open-loop gain schedule in response to estimated vehicle dynamic parameters, the handling feel and directional stability of the original vehicle can be maintained over the lifetime of the vehicle.